The present invention relates to a technology that is advantageously applied to arrangements such as a power source circuit, a power supply system, and an electronic apparatus using an over voltage detection circuit.
The technologies studied by the inventors of the present invention for a power source circuit, a power supply system, and an electronic apparatus include the following. For example, a power supply used for an electronic apparatus is usually equipped with an over voltage detection circuit for protecting the power supply itself and the elements of electronic circuits that are the load.
In general, many electronic apparatuses are configured as shown in FIG. 6. Referring to FIG. 6, an electronic apparatus 10 comprises a power supply system 20 and an electronic circuit 30. The power supply system 20 comprises a DC to DC power supply 100 and an AC to DC power supply 200. The power supply system 20 receives power, such as commercial AC power, from a power receiving terminal 40, uses the AC to DC power supply 200 to convert the received power to DC power, and uses the DC to DC power supply 100 to generate a desired-level of the DC voltage.
FIG. 7 shows the general configuration of a DC to DC power supply. Referring to FIG. 7, the DC to DC power supply 100 comprises a power circuit 110, a control circuit 120, and an over voltage detection circuit 130. The power circuit 110 receives power from input terminals 191 and outputs a desired-level, stabilized DC voltage to output terminals 192. The control circuit 120 turns the power circuit 110 on or off according to a signal received via a control terminal 193. The over voltage detection circuit 130, usually connected to the output terminal 192, outputs a stop signal when the output voltage becomes abnormally high, and stops the operation of the power circuit 110 via, or sometimes not via, the control circuit 120.
One prior-art disclosures dealing with an over voltage detection circuit is found, for example, in JP-A-7-322609, and such a circuit is shown in FIG. 8. In the over voltage detection circuit shown in FIG. 8, terminals 131 and 132 are connected to the output terminals of the power supply. A Zener diode 133 checks if the voltage across the terminals 131 and 132 is equal to or higher than a predetermined setting value and, if the voltage is equal to or higher than the predetermined value, a photo-coupler 134 sends the stop signal. When the power supply fails and the output voltage reaches the predetermined value, this over voltage detection circuit stops the operation of the power supply.
Another example of an over voltage detection circuit is shown in FIG. 9. Referring to FIG. 9, the terminal 131 is connected, via voltage dividing resistors 135 and 136, to a comparator 137 that compares the voltage with the voltage of a reference voltage source 138. If the voltage of the terminal 131 is equal to or higher than a predetermined setting value determined by a reference voltage source 138 and the voltage dividing resistors 135 and 136, the stop signal is output from a signal output terminal 139. When the power supply fails and the output voltage reaches a predetermined value, this over voltage detection circuit stops the operation of the power supply. It should be noted that the voltage dividing resistors 135 and 136 are provided to enable a predetermined voltage, which will be compared, to be set, changed, and adjusted easily.
The over voltage detection circuit described above prevents the output voltage of the power supply from being above a predetermined setting value. It also protects a circuit which is a load from being damaged, and protects the internal components of the power supply itself from being damaged.